1. Field of the Invention
The present invention relates to a discharge tube in which a pair of upper and lower discharge electrodes are vertically arranged, in an airtight cylinder made of insulating material, and are vertically opposed to each other in the axial direction.
2. Description of the Related Art
As shown in FIG. 14, there is provided a discharge tube used for a "Switching Spark Gap (SSG)" for lighting a metal halide lamp and also used as an arrester for preventing generation of a surge voltage.
In this discharge tube, there are provided an upper discharge electrode 20a and a lower discharge electrode 20b, the profiles of which are cylindrical, which are made of a metal such as 42-alloy (iron-nickel alloy) and are arranged in an axial direction in an airtight cylinder 10 made of an insulating material such as a ceramic, and these upper discharge electrode 20a and lower discharge electrode 20b are arranged vertically opposed to each other. Between the fore end surfaces of the upper discharge electrode 20a and lower discharge electrode 20b, there is formed a discharge gap of a predetermined width for generating an electric discharge.
The upper and lower opening ends of the airtight cylinder 10 are airtightly closed by covers 22a, 22b made of a metal such as 42-alloy which are attached to the upper discharge electrode 20a and the lower discharge electrode 20b. The covers 22a, 22b are airtightly joined, by means of soldering, onto metalized layers 12a, 12b which are formed on the upper and lower opening end surfaces of the airtight cylinder 10.
On the inner circumferential wall of the airtight cylinder 10, there are provided a plurality of narrow band-shaped discharge trigger wires 30a, 30b, made of carbon, which are arranged at regular intervals in the lateral direction in parallel with the axis of the airtight cylinder 10.
In the specific structure of the discharge tube, shown in the development view of FIG. 15, a plurality of discharge trigger wires 30a are electrically connected to the upper discharge electrode 20a via the metalized layer 12a in the upper portion of the inner circumferential wall of the airtight cylinder 10. The plurality of discharge trigger wires 30a are arranged in the lateral direction, and the lower end portions of the discharge trigger wires 30a cross the center of the inner circumferential wall of the airtight cylinder 10. A plurality of discharge trigger wires 30b are electrically connected to the lower discharge electrode 20b via the metalized layer 12b in the lower portion of the inner circumferential wall of the airtight cylinder 10. The plurality of discharge trigger wires 30b are arranged in the lateral direction, and upper end portions of the discharge trigger wires 30b cross the center of the inner circumferential wall of the airtight cylinder 10. The plurality of discharge trigger wires 30a, 30b are alternately arranged in the lateral direction on the inner circumferential wall of the airtight cylinder 10.
The discharge tube shown in FIGS. 14 and 15 is composed as described above. In this discharge tube, when a voltage higher than a predetermined value is impressed between the covers 22a, 22b attached to the upper discharge electrode 20a and the lower discharge electrode 20b, it is possible to generate an electrical discharge between the fore end surfaces of the upper discharge electrode 20a and the lower discharge electrode 20b.
At this time, it is possible to generate an initial discharge at an early stage between the end portions of the discharge trigger wires 30a, 30b formed on the inner circumferential wall of the airtight cylinder 10. Due to the initial discharge generated between the end portions of the discharge trigger wires 30a, 30b, it is possible to induce a discharge between the fore end surfaces of the upper discharge electrode 20a and the lower discharge electrode 20b in quick response.
However, the following problems were caused in the above discharge tube. When an electrical discharge was generated between the fore end surfaces of the upper discharge electrode 20a and the lower discharge electrode 20b, a portion of metal composing the upper discharge electrode 20a and the lower discharge electrode 20b was changed into powder and scattered into the surroundings. A sputtering substance 40 was made to adhere to the center of the inner circumferential wall of the airtight cylinder 10 in the lateral direction as shown in FIG. 15. Due to the sputtering substance 40, the electrical insulating property between the discharge trigger wires 30a, 30b, which were arranged on the inner circumferential wall of the airtight cylinder 10, was deteriorated. As a result, it was impossible to precisely generate an initial discharge between the end portions of the discharge trigger wires 30a, 30b over a long period of time.
In order to prevent the deterioration of electrical insulation between the discharge trigger wires 30a, 30b, it is possible to arrange the discharge trigger wires 30a in such a manner that the discharge trigger wires 30a are electrically connected with the upper discharge electrode 20a while they are made to be short and made to come close to the upper end of the inner circumferential wall of the airtight cylinder 10 and that the discharge trigger wires 30b are electrically connected with the lower discharge electrode 20b while they are made to be short and made to come close to the lower end of the inner circumferential wall of the airtight cylinder 10.
According to the above method, it becomes possible to arrange the discharge trigger wires 30a, 30b at positions close to the upper and the lower end of the inner circumferential wall of the airtight cylinder 10 which are distant from the center of the inner circumferential wall of the airtight cylinder 10 to which the sputtering substance 40 scattered from the upper discharge electrode 20a and the lower discharge electrode 20b is made to adhere. Accordingly, it is possible to prevent the deterioration of electrical insulation between the discharge trigger wires 30a, 30b which is caused by the sputtering substance 40 adhering to the center of the inner circumferential wall of the airtight cylinder 10 in the lateral direction, wherein the adhering portion, to which the sputtering substance 40 adheres, at the center of the inner circumferential wall of the airtight cylinder is formed into a band-shape.
However, the above arrangement was disadvantageous in that a distance between the discharge trigger wires 30a, 30b was extended. Therefore, it was impossible to generate an initial discharge at an early stage between the discharge trigger wires 30a, 30b in a stable condition.